A photoacoustic (PA) signal can be obtained when a sample, situated in a gas-filled closed chamber, is illuminated by a beam of light (or other radiation), the intensity of which varies at an acoustic frequency. If the sample absorbs the light, its periodic excitation leads to periodic heating (thermal wave). This wave, when transferred to a thin boundary layer of gas, generates a pressure wave at an acoustic frequency, which is detected by a microphone in the sample chamber. The microphone signal is a measure of that part of the absorbed light that is converted into heat, and, if all the absorbed energy becomes heat (i.e. only thermal decay of the excited states occurs) the PA signal can correspond to the optical absorption by the sample. However, if some of the absorbed light is re-emitted (luminescence) or converted into (electro) chemical or electrical energy, the PA signal can differ significantly from the optical absorption signal, by an amount called "photochemical-loss" (PL) (S. Malkin and D. Cahen, Photochem. Photobiol. 29, 803 (1979)).
Gray et al., Anal. Chem. 50 1262 (1978), have described photoacoustic spectroscopy applied to systems involving photoinduced gas evolution or consumption. Their investigation relates to non-biological samples and their measurements were carried out with a conventional photoacoustic cell. Fishman et al., Anal. Chem. 53, 102 (1981), deal with open-ended photoacoustic spectroscopy cells for thin layer chromatography and other applications. The cell used is one where the sample forms one wall of said cell, and the closure is due to the sheer weight of the cell. The signal to noise ratio is low and such cell is comparatively massive and can only be used with smooth surfaces. The measurements were effected on precoated hard-layer TLC plates.
None of the hitherto known devices was applied to the measurement of photosynthesis of intact photosynthetic tissues in vivo, such as leaves of living plants, algae and bacteria, and hitherto no mobile instrument for this purpose is known.
The only methods tried until now in this respect are thermal radiometry (measurement of the leaf skin temperature by IR detectors), preferably in two dimensions to get a temperature profile of the leaf or plant. A method used nowadays in the laboratory measures the carbon dioxide uptake of plants. It is rather cumbersome and time consuming and not field-adaptable.